Page:Popular Science Monthly Volume 13.djvu/670

650 polished. The whole surface of New York Island, where the rock is exposed, shows marks of glacial action, the upturned edges of gneiss being ground off to form a nearly plane surface, or, where ridges of more massive rock had existed, these are rounded over to form roches moutonnées. Fine examples of the latter may be seen in Central Park and on the east side of the island near Harlem.

The material which occupies so much of the troughs of the Hudson and East River is mostly glacial drift, clay, gravel, sand, and bowlders, scraped from the highlands by the great ice-sheet into these preglacial gorges. It is probable they were once filled to the brim, and that they were subsequently reëxcavated in part by the floods of water which resulted from the melting ice. After these ceased, and they were occupied by water standing at its present level, and moved only by tidal action, they were more or less silted up by the deposit of fine mud brought down by the larger and smaller streams, here checked in their



flow and losing their transporting power. The southern and lower portion of New York Island, which was under the lee of the higher, was covered with deposits left by the retreating glacier, and these were never afterward entirely removed. Here are now beds of sand and gravel which have in places been penetrated to the depth of one hundred feet or more. On the higher parts of the island and the adjacent country, the rock is generally bare or covered with soil, but even here depressions are filled with bowlders, clay, or gravel, often to the depth of several feet, and large transported bowlders are everywhere scattered over the surface. These latter have sometimes been derived from the rocks of the island, but most of them seem to have come from distant points, and always from the north and west. Rounded masses of trap are very common among the bowlders, and these have been brought across the Hudson, for there is no trap in place on the east side of the river. The trap-ledge which forms the summit of the Palisades is everywhere worn and scratched by glacial action, and the markings which it bears are generally concordant in direction with those of the rocks of New York Island and Westchester County, viz., about north-northwest and south-southeast. Even on the river-face of the hills which form the east bank of the Hudson, the bearing of the glacial scratches is essentially the same, showing that the movement of the great ice-sheet was little affected by any such trifling irregularity of the surface beneath it as the Hudson Valley.